Patent GB-1315331-A (GB)
Title: Control systems
IPC: G05D22/02
Filed: 1970-02-05
Abstract: 1315331 Automatic moisture control ENGLISH CALICO Ltd 19 April 1971 [5 Feb 1970 13 March 1970] 5472/70 and 12059/70 Heading G3R  A control system for drying textiles, comprises means for deriving an electrical signal corresponding to a predicted impending error in the moisture content, before the error is reached and means for deriving a correcting signal of duration proportional to the predicted error signal. Textile material web 10 passes through a drying unit 12 and on to a take-up roll 14 via guide rollers 16, 18. Moisture detectors 20, 22 feed D.C. voltages into the control circuit shown in Fig. 2. In Fig. 2, adjustable voltage source 30 to set the degree of dryness required is in opposition to detector 22 so that when the degree of dryness at detector 22 is at the level required the voltages from detector 22 and source 30 cancel each other out. Detector 20 is corrected through capacitor 24 to point 26 so that when the moisture contact at detector 20 is constant no voltage will be felt at point 26 from detector 20 but when the moisture content at detector 20 is charging a voltage will appear at point 26 from detector 20, which voltage will be proportional to the rate of charge of moisture content at detector 20. Thus the error voltage at point 26 predicts the error which will occur when the material at detector 20 reaches detector 22. The signal from point 26 is fed through amplifier 34 into listable comparators 42, 44, then into integrators 48, 52, then fed with a D.C. voltage from the potential divider including variable resistor 58, the outputs from integrators 48, 52 being fed back into comparators 42, 44. Each listable comparator 42, 44 compares the output of amplifier 34 with that of its respective integrator 48, 52 and gives a high or low output as a result of this comparison. The outputs of integrators 48, 52 are set to be initially near zero. For zero output of amplifier 34 the output of comparator 42 is low and that of comparator 44 is high. The operation is as follows. Assume the material reaching detector 20 goes damper than required, the D.C. output of detector 20 goes negative, assume the dampress at detector 22 is at the required level so that detector 22 and source 30 cancel each other out, the input to amplifier 34 is negative, its output going positive as a result. This signal has no effect on comparator 42 but comparator 44 charges from its high to low output state, and the output of integrator 52 starts to follow the error input to comparator 44 until it catches up when comparator 44 reverts to its normal high output state. While comparator 44 was in its low output state, (a) gate 72 produced a decrease speed signal of duration proportional to the error signal to operate decrease speed mechanism 74 to decrease the speed of web 10 through drying unit 12; (b) integrator 48 was reset to zero in gate 76, reset means 60 and gate 64 and an increase speed signal (from gate 68) was inhibited; cycling delay 80 was reset to apply a time delay after which a signal is simultaneously sent to gates 76, 78 to reset integrators 48, 52 to zero so that the cycle can recommense. When the damp cloth reaches detector 22, this gives a negative signal, and the output from detector 20 becomes steady, to give a steady signal from point 26. If the connection was not sufficient a further correction will be made after the time delay produced by delay 80. Delay unit 84 ensures a long delay after the machine has started before a speed increase can occur and delay unit 86 a short delay after starting before a decrease speed can occur. The delays of units 80, 84, 86 are related to throughput time by means of tacho-generator 87. Means 82, which may also be controlled by tachogenerator 87 resets and inhibits comparators 42, 44 when the machine is stopped. United 38 is a meter. An explorating circuit, Fig.3, (not shown) and a practical circuit, Fig. 5, (not shown) are provided for the detectors 20, 22, in which electrodes pass a current from a D. C. source through the web, the strength of the current depending on the degree of dampness, the current being amplified by an amplifier with a chain of diodes and a Zener diode in parallel across it to compensate for the non-linear characteristics of the material. Fig.4, (not shown) gives a thermostatic circuit for controlling the temperature of the diodes in the detector circuits by means of a heating element 200 controlled through transistors by a thermistor, 202.